glsl: Emit errors or warnings when 'layout' is used with 'attribute' or 'varying'

glsl: Emit errors or warnings when 'layout' is used with 'attribute' or 'varying'

The specs that add 'layout' require the use of 'in' or 'out'.
However, a number of implementations, including Mesa, shipped several
of these extensions allowing the use of 'varying' and 'attribute'.
For these extensions only a warning is emitted.

This differs from the behavior of Mesa 7.10.  Mesa 7.10 would only
accept 'attribute' with 'layout(location)'.  This behavior was clearly
wrong.  Rather than carrying the broken behavior forward, we're just
doing the correct thing.

This is related to (piglit) bugzilla #31804.

NOTE: This is a candidate for the 7.9 and 7.10 branches.